Fluorine in a silica glass lowers its refractive index and makes it possible to produce optical fibers having various distribution of refractive indexes and improved light transmission characteristics, for example, an optical fiber having a large difference of refractive index between a core and a cladding and thus a large number of aperture and an optical fiber comprising a core made of pure silica glass and having improved resistance against radiation.
There are known several methods for producing a silica glass type optical fiber preform containing fluorine including a modified chemical vapor deposition (MCVD) method and a plasma outside vapor deposition (POVD) method. Generally, from a preform produced by these method, only about 10 Km of an optical fiber can be drawn. Further, since the production rates of these methods are low, they are not suitable for mass production of the optical fiber preform and thus the optical fiber.
The vapor phase axial deposition method (hereinafter referred to as "VAD" method) can afford an optical fiber preform from which an optical fiber having longer length is drawn. However, the optical fiber preform containing fluorine is rarely produced by the VAD method, since if a fluorine-containing material is simply mixed with gaseous SiCl.sub.4 which is a raw material of SiO.sub.2, there arise many drawbacks such that fluorine is not effectively added to silica glass, the produced preform tends to crack and a deposition rate of fine glass particles is deteriorated.